Spinal deformity surgery is a complex technical procedure that can involve numerous implants and techniques to achieve a straightening of the spine in three dimensions. There is a sagital plane that may contain an increased or decreased lordosis or kyphosis, a coronal plane that may contain a left/right curvature of the spine and an axial plane that may contain a rotational deformity. All three planes must be addressed if a result that leaves the patient functional and provides acceptable cosmesis is to be achieved.
Current techniques involve the use of pedicle screws and various hooks as anchors or fixation points along the posterior spine. Screw types can include polyaxial screws, fixed angle or monoaxial screws and uniplanar screws. Each screw type has a feature set that can be useful in various situations. As such, surgeons will select a screw type for a particular procedure based upon the problems to be addressed and the feature set of the screw type.
Following placement of the fixation points by a physician, the corrective part of the procedure occurs when a rod is introduced into each of the fixation points along the spine. Except in extreme cases, a single “correcting” rod is placed first on one lateral side of the spine, followed by second “stabilizing” rod on the other lateral side of the spine. The rod is contoured to a shape that closely approximates the natural contour of a non-deformed spine. After the rods are introduced into the fixation points, the physician forces the fixation point to meet the rod so that the rod can be locked into the fixation point, which may be a screw or a hook, to hold the spine in a more normal position.
In certain cases, the physician is unable to introduce the rod into all of the fixation points or bring the rod and fixation points together. This can occur if the spine is severely deformed which causes the fixation points to be misaligned. When this occurs, the physician bends the rod in such a way that it can be introduced into the fixation points, but this compromises the correction.
Bending of the rod “in situ” can be used to provide more correction after the rod is seated into all of the fixation points. Obviously, the introduction of a rod into a deformed spine can be very challenging to do so there have been many instruments designed to help forcibly bring the spine and the rod together. Often, when an instrument is used to force the mating of the rod and anchor, the anchor can pull out of the bone because the force is too great. Also, while reducing the rod into one anchor, it may miss the anchor above or below because the alignment of the rod and the spine do not match. In this case, the rod must be released from the anchor and re-introduced taking care, and often another set of hands, to guide the rod into the adjacent anchors during reduction.
Reduction screws have been used widely for some time to aide in the introduction of a rod into a fixation point and forcing of the rod into a seat of the screw. Reduction screws have extended arms or tabs that are threaded so that a set screw can be introduced into the extended tabs for securing the rod into the screw before the rod is fully seated in the screw seat. By turning the set screw, the rod is driven down the extension tabs and into the screw seat of the screw head in a controlled and progressive manner.
A shortcoming of current reduction screws is that a physician utilizing a reduction screw must turn the set screw ten to fifteen complete turns to seat the rod. Since it is not uncommon for twenty or more screws to be used during a corrective spine surgery, reduction screws are underused as physicians are reluctant to invest the time and effort required to turn the set screws three hundred times. Another shortcoming of reduction screws is that their extension tabs rarely extend longer than approximately one inch. This is in part due to the additional set screw advancement, i.e., complete set screw turns, required when longer extension tabs are used. Further, longer extension tabs are prone to “splay” when reduction forces are applied. This can cause the set screw to disengage or cross-thread. Instruments have been designed to control the splaying effect, but they tend to complicate the procedure. Yet another shortcoming of reduction screws is that the amount of torque they require to drive a rod into the screw seat is difficult to translate into actual downward force by the physician. A few inch pounds of torque created by rotating a screwdriver handle can equate to over one hundred pounds of force on the screw. This can cause the pedicle screw to pull out of the bone thus causing the loss of a fixation point.